Improved update() docs in querysets.txt

git-svn-id: http://code.djangoproject.com/svn/django/trunk@16516 bcc190cf-cafb-0310-a4f2-bffc1f526a37

docs/ref/models/querysets.txt

@@ -1278,24 +1278,65 @@ update
 .. method:: update(**kwargs)
 
 Performs an SQL update query for the specified fields, and returns
-the number of rows affected. The ``update()`` method is applied instantly and
+the number of rows affected.
-the only restriction on the :class:`.QuerySet` that is updated is that it can
-only update columns in the model's main table. Filtering based on related
-fields is still possible. You cannot call ``update()`` on a
-:class:`.QuerySet` that has had a slice taken or can otherwise no longer be
-filtered.
 
-For example, if you wanted to update all the entries in a particular blog
+For example, to turn comments off for all blog entries published in 2010,
-to use the same headline::
+you could do this::
 
-    >>> b = Blog.objects.get(pk=1)
+    >>> Entry.objects.filter(pub_date__year=2010).update(comments_on=False)
 
-    # Update all the headlines belonging to this Blog.
+(This assumes your ``Entry`` model has fields ``pub_date`` and ``comments_on``.)
-    >>> Entry.objects.select_related().filter(blog=b).update(headline='Everything is the same')
 
-The ``update()`` method does a bulk update and does not call any ``save()``
+You can update multiple fields -- there's no limit on how many. For example,
-methods on your models, nor does it emit the ``pre_save`` or ``post_save``
+here we update the ``comments_on`` and ``headline`` fields::
-signals (which are a consequence of calling ``save()``).
+
+    >>> Entry.objects.filter(pub_date__year=2010).update(comments_on=False, headline='This is old')
+
+The ``update()`` method is applied instantly, and the only restriction on the
+:class:`.QuerySet` that is updated is that it can only update columns in the
+model's main table, not on related models. You can't do this, for example::
+
+    >>> Entry.objects.update(blog__name='foo') # Won't work!
+
+Filtering based on related fields is still possible, though::
+
+    >>> Entry.objects.filter(blog__id=1).update(comments_on=True)
+
+You cannot call ``update()`` on a :class:`.QuerySet` that has had a slice taken
+or can otherwise no longer be filtered.
+
+The ``update()`` method returns the number of affected rows::
+
+    >>> Entry.objects.filter(id=64).update(comments_on=True)
+    1
+
+    >>> Entry.objects.filter(slug='nonexistent-slug').update(comments_on=True)
+    0
+
+    >>> Entry.objects.filter(pub_date__year=2010).update(comments_on=False)
+    132
+
+If you're just updating a record and don't need to do anything with the model
+object, you should use ``update()`` rather than loading the model object into
+memory. The former is more efficient. For example, instead of doing this::
+
+    e = Entry.objects.get(id=10)
+    e.comments_on = False
+    e.save()
+
+...do this::
+
+    Entry.objects.get(id=10).update(comments_on=False)
+
+Finally, note that the ``update()`` method does an update at the SQL level and,
+thus, does not call any ``save()`` methods on your models, nor does it emit the
+``pre_save`` or ``post_save`` signals (which are a consequence of calling
+``save()``). If you want to update a bunch of records for a model that has a
+custom ``save()`` method, loop over them and call ``save()``, like this::
+
+    for e in Entry.objects.filter(pub_date__year=2010):
+        e.comments_on = False
+        e.save()
 
 delete
 ~~~~~~